This Application relates to a transmission for driving a vehicle transfer case actuator wherein a cushion is provided for a stop which prevents movement of the transmission beyond desired ends of travel.
In the prior art, transfer cases allow vehicles to be switched between two-wheel and four-wheel drive modes of operation. In addition, transfer cases typically allow both high and low gear ratios in both two- and four-wheel drive modes.
It has been standard that an actuator assembly moves shift collars between locations to drive one or two axles and engage either the high or low gear ratios. The actuator assembly has typically included a motor driving a worm. The worm engages a worm gear, and a shaft is driven by the worm gear. An actuator pin is driven by the shaft between several positions at which the various drive modes are obtained.
In one example, the worm drives the worm gear through a range of approximately 270.degree.. A groove in the driven shaft moves an actuator pin between any one of four distinct axial positions. In the distinct axial positions, the actuator pin moves shift collars between locations wherein one axle or two axles are driven, and also between high and low gear ratios. Thus, by selectively rotating the worm gear, one can move the actuator pin to a desired location and achieve a particular drive mode. The motor is preferably a reversible motor such that the pin can be moved back and forth between the positions.
Problems have arisen in the prior art with such transmission. In particular, it would be undesirable to allow the shaft to rotate 360.degree.. Such rotation could lock the transfer case members and prevent movement of the shift collars or actuator pin. To that end, stops have typically been provided on the worm gears to prevent rotation from approaching 360.degree.. In one prior art example, a stop pin extends axially from a face of the worm gear and contacts a stop surface within the worm gear housing. In this way, the stop pin prevents the gear from even approaching 360.degree. of rotation.
In the prior art systems, a smaller degree of angular motion is typically required to achieve the required movement of the actuator pin. In theory the worm gear should never approach the extremes of the stop. As one example, in the prior art systems, only 270.degree. of rotation is necessary to actuate or move the actuator pin, while the motor transmission has 330.degree. of range. A position feedback sensor monitors the position of the worm gear and should stop rotation at the extremes of the 270.degree. range. However, in practice it does happen that the motor occasionally moves beyond the 270.degree. range. In such circumstances, without a stop, the motor would extend up to 360.degree. of rotation. The motors typically incorporated into the use systems allow full rotation, as a limited rotation motor is unduly expensive. For that reason, the prior art has typically included a stop to prevent rotation beyond a greater range (i.e. 330.degree. of rotation). Thus, as the transmission approaches 330.degree. of rotation, the stop will prevent further rotation. In the past, the stop often wedges into the housing structure and locks the gear motor. Once this happens, the system is no longer functional.